Are they sent in one massive batch to the email server or sent in batches etc?
nopCommerce sends email to the email server in batches.
How are they sent?
For example, when you have created a campaign and clicked the "Send mass email" button, nopCommerce will create a individual message for each subscriber by replacing message tokens, that messages will be placed in the message queue. The message queue is handled by the SendQueuedMessagesTask task, which is invoking one time per 120 seconds(by default).
You can change it by yourself by editing ScheduleTasks section of the web.config file. Also, be aware that application is started not permanently, so don't specify a too long time.